Lesson 11.4: Integration of Time-of-Flight Distance Sensors for Obstacle Detection

How Time-of-Flight Ranging Works

A time-of-flight sensor emits a laser pulse and measures how long the pulse takes to return after bouncing off a surface. Because light travels at a fixed and known speed, the elapsed time maps directly and precisely to a distance. The sensor does not need to be calibrated for different materials the way an ultrasonic sensor does, and it is far less susceptible to ambient sound than ultrasonic alternatives.

The REV Color/Distance Sensor integrates this ranging capability alongside the color sensor covered in Lesson 11.3. One physical device, one I2C connection, and one configuration name in the Robot Controller app provide both capabilities at the same time. This is one of the most cost-effective sensor combinations available in FTC.

The DistanceSensor Interface and Unit Handling

In the SDK, distance sensing functionality is accessed through the DistanceSensor interface. The method getDistance(DistanceUnit unit) returns the measured distance as a double. The DistanceUnit parameter is an enum with values including CM, MM, INCH, and METER. Passing the unit directly into the method means the SDK performs all conversion math internally, and the value you receive is already in the unit you requested.

Because the REV sensor is one physical device that satisfies two different Java interfaces, you can retrieve it from the hardwareMap using the same configuration name for both ColorSensor and DistanceSensor. The SDK resolves both references to the same underlying hardware object transparently.

ColorSensor    colorSensor = hardwareMap.get(ColorSensor.class,    "front_sensor");
DistanceSensor rangeSensor = hardwareMap.get(DistanceSensor.class, "front_sensor");

Both variables now reference the same physical device, and you can use them independently throughout your code.

Practical Limits and Validation

The time-of-flight sensor on the REV module is accurate roughly between 1 cm and 200 cm. Objects that are very close, very far away, or made of dark matte materials that absorb the laser rather than reflect it can produce unreliable readings. The SDK returns Double.MAX_VALUE when the sensor cannot obtain a valid measurement. Checking for this before using a distance value in control logic prevents erratic motor behavior caused by a stray bad reading.

double dist = rangeSensor.getDistance(DistanceUnit.CM);
if (dist < 200.0) { // Only act on readings within a believable range
    // control logic here
}

Annotated Code

package org.firstinspires.ftc.teamcode;

import com.qualcomm.robotcore.eventloop.opmode.LinearOpMode;
import com.qualcomm.robotcore.eventloop.opmode.TeleOp;
import com.qualcomm.robotcore.hardware.DcMotor;
import com.qualcomm.robotcore.hardware.DistanceSensor;
import org.firstinspires.ftc.robotcore.external.navigation.DistanceUnit;

@TeleOp(name="Distance_Sensor_Demo")
public class DistanceSensorDemo extends LinearOpMode {

    @Override
    public void runOpMode() {
        DcMotor        driveMotor = hardwareMap.get(DcMotor.class,     "left_drive");
        DistanceSensor frontRange = hardwareMap.get(DistanceSensor.class, "front_sensor");

        driveMotor.setZeroPowerBehavior(DcMotor.ZeroPowerBehavior.BRAKE);

        waitForStart();

        while (opModeIsActive()) {
            double distCm = frontRange.getDistance(DistanceUnit.CM);

            double power = -gamepad1.left_stick_y;

            // Validate the reading before using it to make a safety decision
            if (distCm < 200.0 && distCm < 15.0 && power > 0) {
                // Robot is within 15 cm of an obstacle -- block forward movement
                power = 0.0;
                telemetry.addData("Warning", "OBSTACLE DETECTED");
            }

            driveMotor.setPower(power);
            telemetry.addData("Distance (cm)", "%.1f", distCm);
            telemetry.addData("Power",         power);
            telemetry.update();
        }
    }
}

Fill-in-the-Blank Practice

To retrieve a distance reading in centimeters, pass DistanceUnit.__________ as the argument to getDistance().
The Java interface used for laser-based distance measurement in the FTC SDK is __________.
When the REV distance sensor cannot obtain a valid measurement, it returns the value Double.__________.

Show answers

CM
DistanceSensor
MAX_VALUE

Template Challenge

Robot Scenario: You need to stop a "drive" motor automatically if the robot comes within 12.0 cm of an obstacle detected by a DistanceSensor named "front_range". If the reading is valid and within 12.0 cm, cut motor power. Otherwise apply the normal joystick value.

package org.firstinspires.ftc.teamcode;

import com.qualcomm.robotcore.eventloop.opmode.LinearOpMode;
import com.qualcomm.robotcore.eventloop.opmode.TeleOp;
import com.qualcomm.robotcore.hardware.DcMotor;
import com.qualcomm.robotcore.hardware.DistanceSensor;
import org.firstinspires.ftc.robotcore.external.navigation.DistanceUnit;

@TeleOp(name="Obstacle_Stop_Challenge")
public class ObstacleStop extends LinearOpMode {

    @Override
    public void runOpMode() {
        DcMotor        drive      = hardwareMap.get(DcMotor.class,        "drive");
        DistanceSensor frontRange = hardwareMap.get(DistanceSensor.class, "front_range");

        waitForStart();

        while (opModeIsActive()) {
            // INSERT CODE HERE: Get the distance in CM
            double distCm = // INSERT VALUE HERE

            double power = -gamepad1.left_stick_y;

            // INSERT CODE HERE: If a valid reading is under 12.0 cm, set power to 0
            
            drive.setPower(power);
            telemetry.addData("Distance (cm)", "%.1f", distCm);
            telemetry.update();
        }
    }
}

Show answer

double distCm = frontRange.getDistance(DistanceUnit.CM);

if (distCm < 200.0 && distCm < 12.0) {
    power = 0.0;
}

Ready to move on?

How Time-of-Flight Ranging Works​

The DistanceSensor Interface and Unit Handling​

Practical Limits and Validation​

Annotated Code​

Fill-in-the-Blank Practice​

Template Challenge​